UPDATE:
Company Email:
"The BJ's beers on tap in our
restaurants are free of animal products, even the dark beers. The only
exceptions are the cask ales that are served in a few of our brewery
locations. We use finings in the cask ales, whereas our draft beers are
either filtered or unfiltered with no use of finings."

Company Email:
"BJ’s draft beers do not contain any animal-derived products. The only exception to this rule is our cask ales, which are clarified with isinglass. But we only serve cask ales in a few of our breweries outside of Texas."